Please wait a minute...
Shopping Cart Email List Chinese
Advanced Search Fig/Tab
Frontiers of Optoelectronics

ISSN 2095-2759

ISSN 2095-2767(Online)

CN 10-1029/TN

Postal Subscription Code 80-976

Featured Articles  |  Most Downloaded  |  Most Reading
Online Submission Subscribe to Our RSS Content Feed
Front. Optoelectron.    2018, Vol. 11 Issue (4) : 375-384    https://doi.org/10.1007/s12200-018-0855-4
RESEARCH ARTICLE
Efficient deep red phosphorescent OLEDs using 1,2,4-thiadiazole core-based novel bipolar host with low efficiency roll-off
Runda GUO, Wenzhi ZHANG, Qing ZHANG, Xialei LV, Lei WANG()
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
 Download: PDF(468 KB)   HTML
 Export: BibTeX | EndNote | Reference Manager | ProCite | RefWorks
Abstract

A series of 1,2,4-thiadiazole core-based bipolar materials, 2,2'-(1,2,4-thiadiazole-3,5-diyl)bis(N,N-diphenylaniline) (o-TPATHZ), 3,3′-(1,2,4-thiadiazole-3,5-diyl)bis(N,N-diphenylaniline) (m-TPATHZ) and 4,4'-(1,2,4-thiadiazole-3,5-diyl)bis(N,N-diphenylaniline) (p-TPATHZ) were developed as the host matrixes for the deep red phosphorescent emitters tris(1-phenylisoqiunoline)iridium (Ir(piq)3) and [bis(2-methyldibenzo-[f,h]-quinoxaline)Ir(III)(acetylacetonate)] (Ir(MDQ)2(acac)). By systematic studying, we demonstrated that there are two types of charge-trapping effect within the emissive layers through adjusting the host-guest compatibility. And, it is revealed that a symmetric charge-trapping effect can contribute to realizing a stable charge-balance, which led to a mitigating efficiency roll-off at high current density. Consequently, a maximum external quantum efficiency (EQE) of 16.2% was achieved by an optimized device with p-TPATHZ-Ir(piq)3 emissive layer. Remarkably, the EQE still remained as high as 15.7% at the high luminance of 1000 cd/m2.
Keywords 1      2      4-thiadiazole core      low efficiency roll-off      deep red phosphorescent devices      symmetrical charge-trapping effect     
Corresponding Author(s): Lei WANG   
Just Accepted Date: 01 November 2018   Online First Date: 13 December 2018    Issue Date: 21 December 2018
 Cite this article:   
Runda GUO,Wenzhi ZHANG,Qing ZHANG, et al. Efficient deep red phosphorescent OLEDs using 1,2,4-thiadiazole core-based novel bipolar host with low efficiency roll-off[J]. Front. Optoelectron., 2018, 11(4): 375-384.
 URL:  
https://academic.hep.com.cn/foe/EN/10.1007/s12200-018-0855-4
https://academic.hep.com.cn/foe/EN/Y2018/V11/I4/375
Fig.1  Scheme 1 Synthetic routes and chemical structures of the materials. Reaction condition: Pd(PPh3)4, K2CO3 (2.0 mol/L), toluene, ethanol 100°C
Fig.2  (a) Differential scanning calorimetry (DSC) thermograms of o-TPATHZ, m-TPATHZ, and p-TPATHZ; (b) thermogravimetric analysis (TGA) curves of o-TPATHZ, m-TPATHZ, and p-TPATHZ recorded at a heating rate of 10°C/min
Fig.3  Ultraviolet-visible (UV-Vis) absorption and PL spectra of o-TPATHZ, m-TPATHZ, and p-TPATHZ in dichloromethane (DCM). RT: room temperature
Fig.4  PL spectra of o-TPATHZ, m-TPATHZ, and p-TPATHZ in 2-methyltetrahydrofuran at 77 K
Fig.5  Cyclic voltammograms of o-TPATHZ, m-TPATHZ, and p-TPATHZ at room temperature with a scan rate of 100 mV/s
Fig.6  Calculated HOMO/LUMO distributions and energy levels of the host materials. Computational Details: The geometrical and electronic properties were computed using the Gaussian 09 program package. Molecular orbitals were visualized using Gaussian view. The calculation was optimized by means of the B3LYP (Becke three parameters hybrid functional with Lee-Yang-Perdew correlation functional) [39] with the 6-31G (d) atomic basis set. Then the electronic structures were calculated at t-HCTHhyb/6–311+ + G (d, p) level [40]. Molecular orbitals were visualized using Gaussian view
compounds Absa
/nm		 PLa
/nm		 Td/Tgb
/°C		 Egc
/eV		 ETd
/eV		 Eoxe
/V		 (HOMO/
LUMO)/eVe		 (HOMO/
LUMO)/eVf
o-TPATHZ 374 503 335/76 2.95 2.68 0.71 –5.36/2.41 –1.69/–4.86
m-TPATHZ 376 512 365/69 2.93 2.64 0.75 –5.41/–2.48 –1.78/–4.90
p-TPATHZ 370 490 402/88 2.97 2.41 0.71 –5.37/–2.40 –1.63/–4.86
Tab.1  Physical properties of o-TPATHZ, m-TPATHZ, and p-TPATHZ
Fig.7  Configuration of the devices based on o-TPATHZ, m-TPATHZ, and p-TPATHZ and the energy level diagram of all the materials used
Fig.8  EQE-brightness properties of devices (a) A1–C1 and (b) A2–C2
device host guest Von
/V		 [hc ]a
/(cd·A−1)		 [hp]b
/(lm·W−1)		 [hEQE]c
/%		 roll-offd
/%		 CIE [x, y]e
A1 o-TPATHZ Ir(MDQ)2(acac) 3.2 28.0 26.2 15.2/12.3/9.5 19.1/37.5 (0.60, 0.40)
B1 m-TPATHZ Ir(MDQ)2(acac) 3.6 24.3 19.7 13.8/12.8/10.0 7.2/27.5 (0.59, 0.39)
C1 p-TPATHZ Ir(MDQ)2(acac) 3.5 25.8 24.0 17.4/13.0/10.3 25.3/40.8 (0.61, 0.38)
A2 o-TPATHZ Ir(piq)3 3.5 11.1 10.8 14.4/14.2/12.8 1.4/11.1 (0.68, 0.32)
B2 m-TPATHZ Ir(piq)3 3.1 9.4 9.3 11.6/11.5/10.4 0.9/10.3 (0.67, 0.33)
C2 p-TPATHZ Ir(piq)3 2.9 13.3 13.6 16.2/16.2/15.7 0/3.1 (0.68, 0.32)
Tab.2  EL data of devices A1–C1 and A2–C2
Fig.9  EL spectra of devices (a) C1 and (b) C2. The inserts are the EL spectra when the wavelength are from 380 to 520 nm
Fig.10  Current density versus voltage curves of the hole-only devices incorporating p-TPATHZ and (a) Ir(piq)3; (b) Ir(MDQ)2(acac)
Fig.11  Current density versus voltage curves of the electron-only devices incorporating p-TPATHZ and (a) Ir(piq)3; (b) Ir(MDQ)2(acac)
device host guest [hEQE ]a/% [hEQE]b/% roll-offc/%
Ref. [43] TCPB Ir(piq)3 18.6 10.2/7.93 45.2/57.4
Ref. [43] TCPY Ir(piq)3 18.4 15.5/13.8 15.8/25
Ref. [43] TCPM Ir(piq)3 18.2 17.8/15.4 2.2/15.4
Ref. [44] 1d Ir(piq)3 18.2 13.6/8.36 25.3/54.1
Ref. [44] 2d Ir(piq)3 15.8 12.6/9.9 20.3/37.3
Ref. [44] 3d Ir(piq)3 15.7 13.2/11.6 15.9/26.1
A2 o-TPATHZ Ir(piq)3 14.4 14.2/12.8 1.4/11.1
B2 m-TPATHZ Ir(piq)3 11.6 11.5/10.4 0.9/10.3
C2 p-TPATHZ Ir(piq)3 16.2 16.2/15.7 0/3.1
Tab.3  Compare the efficiency roll-offs with reported Ir(piq)3-based OLEDs
1 S RForrest. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature, 2004, 428(6986): 911–918
https://doi.org/10.1038/nature02498 pmid: 15118718
2 HSasabe, J Kido. Development of high performance OLEDs for general lighting. Journal of Materials Chemistry C, Materials for Optical and Electronic Devices, 2013, 1(9): 1699–1707
https://doi.org/10.1039/c2tc00584k
3 SReineke, F Lindner, GSchwartz, NSeidler, KWalzer, BLüssem, KLeo. White organic light-emitting diodes with fluorescent tube efficiency. Nature, 2009, 459(7244): 234–238
https://doi.org/10.1038/nature08003 pmid: 19444212
4 LXiao, Z Chen, BQu, JLuo, S Kong, QGong, JKido. Recent progresses on materials for electrophosphorescent organic light-emitting devices. Advanced Materials, 2011, 23(8): 926–952
https://doi.org/10.1002/adma.201003128 pmid: 21031450
5 YMa, H Zhang, JShen, CChe. Electroluminescence from triplet metal-ligand charge-transfer excited state of transition metal complexes. Synthetic Metals, 1998, 94(3): 245–248
https://doi.org/10.1016/S0379-6779(97)04166-0
6 M ABaldo, D F O’brien, Y You, AShoustikov, SSibley, M EThompson, S RForrest. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature, 1998, 395(6698): 151–154
https://doi.org/10.1038/25954
7 XYang, G Zhou, W YWong. Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices. Chemical Society Reviews, 2015, 44(23): 8484–8575
https://doi.org/10.1039/C5CS00424A pmid: 26245654
8 SZhuang, W Zhang, XYang, LWang. A simple unilateral homogenous PhOLEDs with enhanced efficiency and reduced efficiency roll-off. Frontiers of Optoelectronics, 2013, 6(4): 435–439
https://doi.org/10.1007/s12200-013-0349-3
9 AHussain, T Zerin, M AKhan. Design and simulation to improve the structural efficiency of green light emission of GaN/InGaN/AlGaN light emitting diode. Frontiers of Optoelectronics, 2017, 10(4): 370–377
https://doi.org/10.1007/s12200-017-0705-9
10 XHu, X Xia, LZhou, LZhang, WWu. Design of compensation pixel circuit with In-Zn-O thin film transistor for active-matrix organic light-emitting diode 3D display. Frontiers of Optoelectronics, 2017, 10(1): 45–50
https://doi.org/10.1007/s12200-016-0562-y
11 M ABaldo, S Lamansky, P EBurrows, M EThompson, S RForrest. Very high-efficiency green organic light-emitting devices based on electrophosphorescence. Applied Physics Letters, 1999, 75(1): 4–6
https://doi.org/10.1063/1.124258
12 K HKim, S Lee, C KMoon, S YKim, Y SPark, J HLee, JWoo Lee, JHuh, Y You, J JKim. Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes. Nature Communications, 2014, 5(1): 4769–4777
https://doi.org/10.1038/ncomms5769 pmid: 25204981
13 JChen, F Zhao, DMa. Hybrid white OLEDs with fluorophors and phosphors. Materials Today, 2014, 17(4): 175–183
https://doi.org/10.1016/j.mattod.2014.04.002
14 CXiang, W Koo, FSo, HSasabe, JKido. A systematic study on efficiency enhancements in phosphorescent green, red and blue microcavity organic light emitting devices. Light, Science & Applications, 2013, 2(6): 74–81
https://doi.org/10.1038/lsa.2013.30
15 S OJeon, S E Jang, H S Son, J Y Lee. External quantum efficiency above 20% in deep blue phosphorescent organic light-emitting diodes. Advanced Materials, 2011, 23(12): 1436–1441
https://doi.org/10.1002/adma.201004372 pmid: 21433109
16 C WLee, J Y Lee. Above 30% external quantum efficiency in blue phosphorescent organic light-emitting diodes using pyrido[2,3-b]indole derivatives as host materials. Advanced Materials, 2013, 25(38): 5450–5454
https://doi.org/10.1002/adma.201301091 pmid: 23788128
17 CAdachi, M A Baldo, M E Thompson, S R Forrest. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. Journal of Applied Physics, 2001, 90(10): 5048–5051
https://doi.org/10.1063/1.1409582
18 Y TTao, Q Wang, C LYang, CZhong, J GQin, D GMa. Multifunctional triphenylamine/oxadiazole hybrid as host and exciton‐blocking material: high efficiency green phosphorescent OLEDs using easily available and common materials. Advanced Functional Materials, 2010, 20(17): 2923–2929
https://doi.org/10.1002/adfm.201000669
19 C HFan, P Sun, T HSu, C HCheng. Host and dopant materials for idealized deep-red organic electrophosphorescence devices. Advanced Materials, 2011, 23(26): 2981–2985
https://doi.org/10.1002/adma.201100610 pmid: 21567483
20 J HJou, Y T Su, M T Hsiao, H H Yu, Z K He, S C Fu, C H Chiang, C T Chen, C H Chou, J J Shyue. Solution-process-feasible deep-red phosphorescent emitter. Journal of Physical Chemistry C, 2016, 120(33): 18794–18802
https://doi.org/10.1021/acs.jpcc.6b07740
21 J VCaspar, T J Meyer. Application of the energy gap law to nonradiative, excited-state decay. Journal of Physical Chemistry, 1983, 87(6): 952–957
https://doi.org/10.1021/j100229a010
22 D HKim, N S Cho, H Y Oh, J H Yang, W S Jeon, J S Park, M C Suh, J H Kwon. Highly efficient red phosphorescent dopants in organic light-emitting devices. Advanced Materials, 2011, 23(24): 2721–2726
https://doi.org/10.1002/adma.201100405 pmid: 21495090
23 BJiang, Y Gu, J JQin, X WNing, S LGong, G HXie, C LYang. Deep-red iridium (III) complexes cyclometalated by phenanthridine derivatives for highly efficient solution-processed organic light-emitting diodes. Journal of Materials Chemistry C, Materials for Optical and Electronic Devices, 2016, 4(16): 3492–3498
https://doi.org/10.1039/C6TC00148C
24 WCho, G Sarada, HLee, MSong, Y S Gal, Y Lee, S HJin. Highly efficient, conventional and flexible deep-red phosphorescent OLEDs using ambipolar thiophene/selenophene-phenylquinoline ligand-based Ir(III) complexes. Dyes and Pigments, 2017, 136: 390–397
https://doi.org/10.1016/j.dyepig.2016.08.060
25 S HCheng, W Y Hung, M H Cheng, H F Chen, G H Lee, C L Chung, T C Yeh, W C Tang, S L Huang, K T Wong. Highly twisted carbazole-oxadiazole hybrids as universal bipolar hosts for high efficiency PhOLEDs. Advanced Electronic Materials, 2016, 2(1): 1500241
https://doi.org/10.1002/aelm.201500241
26 LWang, B Pan, LZhu, BWang, Y Wang, YLiu, JJin, J Chen, DMa. Construction of thermally stable 3,6-disubstituted spiro-fluorene derivatives as host materials for blue phosphorescent organic light-emitting diodes. Dyes and Pigments, 2015, 114: 222–230
https://doi.org/10.1016/j.dyepig.2014.11.011
27 S YByeon, J Y Lee. High-triplet-energy host materials derived from directly-coupled carbazole-pyridoindole moieties. Dyes and Pigments, 2016, 130: 183–190
https://doi.org/10.1016/j.dyepig.2016.03.004
28 C WSeo, J H Yoon, J Y Lee. Engineering of charge transport materials for universal low optimum doping concentration in phosphorescent organic light-emitting diodes. Organic Electronics, 2012, 13(2): 341–349
https://doi.org/10.1016/j.orgel.2011.11.007
29 YLuo, H Aziz. Correlation between triplet-triplet annihilation and electroluminescence efficiency in doped fluorescent organic light-emitting devices. Advanced Functional Materials, 2010, 20(8): 1285–1293
https://doi.org/10.1002/adfm.200902329
30 YKawamura, J Brooks, J JBrown, HSasabe, CAdachi. Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir(III) complexes in a solid film. Physical Review Letters, 2006, 96(1): 017404
https://doi.org/10.1103/PhysRevLett.96.017404 pmid: 16486515
31 YTao, Q Wang, CYang, QWang, Z Zhang, TZou, JQin, D Ma. A simple carbazole/oxadiazole hybrid molecule: an excellent bipolar host for green and red phosphorescent OLEDs. Angewandte Chemie, 2008, 47(42): 8104–8107
https://doi.org/10.1002/anie.200803396 pmid: 18798180
32 CLi, L Duan, H YLi, YQiu. Universal trap effect in carrier transport of disordered organic semiconductors: transition from shallow trapping to deep trapping. Journal of Physical Chemistry C, 2014, 118(20): 10651–10660
https://doi.org/10.1021/jp5022906
33 CLi, L Duan, Y DSun, H YLi, YQiu. Charge transport in mixed organic disorder semiconductors: trapping, scattering, and effective energetic disorder. Journal of Physical Chemistry C, 2012, 116(37): 19748–19754
https://doi.org/10.1021/jp307951h
34 W ZZhang, J J Jin, Z Huang, XLv, SZhuang, LWang. Towards highly efficient thermally activated delayed fluorescence devices through a trap-assisted recombination mechanism and reduced interfacial exciton annihilation. Journal of Materials Chemistry C, Materials for Optical and Electronic Devices, 2017, 5(19): 4636–4644
https://doi.org/10.1039/C7TC00653E
35 FAlonso, I P Beletskaya, M Yus. Non-conventional methodologies for transition-metal catalysed carbon–carbon coupling: a critical overview. Part 2: The Suzuki reaction. Tetrahedron, 2008, 64(14): 3047–3101
https://doi.org/10.1016/j.tet.2007.12.036
36 J JJin, W Z Zhang, B Wang, G YMu, PXu, L Wang, HHuang, J SChen, D GMa. Construction of high Tg bipolar host materials with balanced electron–hole mobility based on 1, 2, 4-thiadiazole for phosphorescent organic light-emitting diodes. Chemistry of Materials, 2014, 26(7): 2388–2395
https://doi.org/10.1021/cm403388s
37 CFan, Y Chen, ZLiu, ZJiang, CZhong, DMa, J Qin, CYang. Tetraphenylsilane derivatives spiro-annulated by triphenylamine/carbazole with enhanced HOMO energy levels and glass transition temperatures without lowering triplet energy: host materials for efficient blue phosphorescent OLEDs. Journal of Materials Chemistry C, Materials for Optical and Electronic Devices, 2013, 1(3): 463–469
https://doi.org/10.1039/C2TC00082B
38 D RLee, J M Choi, C W Lee, J Y Lee. Ideal molecular design of blue thermally activated delayed fluorescent emitter for high efficiency, small singlet–triplet energy splitting, low efficiency roll-off, and long lifetime. ACS Applied Materials & Interfaces, 2016, 8(35): 23190–23196
https://doi.org/10.1021/acsami.6b05877 pmid: 27529181
39 CLee, W Yang, R GParr. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Physical Review B, 1988, 37(2): 785–789
https://doi.org/10.1103/PhysRevB.37.785 pmid: 9944570
40 A DBoese, N C Handy. New exchange-correlation density functionals: the role of the kinetic-energy density. Journal of Chemical Physics, 2002, 116(22): 9559–9569
https://doi.org/10.1063/1.1476309
41 Y LChang, Z B Wang, M G Helander, J Qiu, D PPuzzo, Z HLu. Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting. Organic Electronics, 2012, 13(5): 925–931
https://doi.org/10.1016/j.orgel.2012.01.026
42 W SJeon, T J Park, S Y Kim, R Pode, JJang, J HKwon. Ideal host and guest system in phosphorescent OLEDs. Organic Electronics, 2009, 10(2): 240–246
https://doi.org/10.1016/j.orgel.2008.11.012
43 S JSu, C Cai, JKido. Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes. Journal of Materials Chemistry, 2012, 22(8): 3447–3456
https://doi.org/10.1039/c2jm14151e
44 S JSu, C Cai, JKido. RGB phosphorescent organic light-emitting diodes by using host materials with heterocyclic cores: effect of nitrogen atom orientations. Chemistry of Materials, 2011, 23(2): 274–284
https://doi.org/10.1021/cm102975d
[1] Qixin WAN, Hao LIN, Shuai WANG, Jiangnan DAI, Changqing CHEN. Ripening-resistance of Pd on TiO2(110) from first-principles kinetics[J]. Front. Optoelectron., 2020, 13(4): 409-417.
[2] Junze LI, Haizhen WANG, Dehui LI. Self-trapped excitons in two-dimensional perovskites[J]. Front. Optoelectron., 2020, 13(3): 225-234.
[3] Yuhan YAO, Zhao CHENG, Jianji DONG, Xinliang ZHANG. Performance of integrated optical switches based on 2D materials and beyond[J]. Front. Optoelectron., 2020, 13(2): 129-138.
[4] Oliver SALE, Safaa HASSAN, Noah HURLEY, Khadijah ALNASSER, Usha PHILIPOSE, Hualiang ZHANG, Yuankun LIN. Holographic fabrication of octagon graded photonic super-crystal and potential applications in topological photonics[J]. Front. Optoelectron., 2020, 13(1): 12-17.
[5] Chunxiang ZENG, Zeqing WANG, Yingmao XIE. Transmission characteristics of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals[J]. Front. Optoelectron., 2019, 12(4): 365-371.
[6] Neha JAIN, O. P. SINHA, Sujata PANDEY. Optimization of organic light emitting diode for HAT-CN based nano-structured device by study of injection characteristics at anode/organic interface[J]. Front. Optoelectron., 2019, 12(3): 268-275.
[7] Peng LI, Sheng LIU, Yi ZHANG, Lei HAN, Dongjing WU, Huachao CHENG, Shuxia QI, Xuyue GUO, Jianlin ZHAO. Modulation of orbital angular momentum on the propagation dynamics of light fields[J]. Front. Optoelectron., 2019, 12(1): 69-87.
[8] Xiaoyan HU, Heng WANG. ZnO/Nb2O5 core/shell nanorod array photoanode for dye-sensitized solar cells[J]. Front. Optoelectron., 2018, 11(3): 285-290.
[9] Chenghong WU, Xinyang MIAO, Kun ZHAO. Identifying PM2.5 samples collected in different environment by using terahertz time-domain spectroscopy[J]. Front. Optoelectron., 2018, 11(3): 256-260.
[10] Tieshan YANG, Han LIN, Baohua JIA. Two-dimensional material functional devices enabled by direct laser fabrication[J]. Front. Optoelectron., 2018, 11(1): 2-22.
[11] Yonglun TANG, Haibo REN, Jiarui HUANG. Synthesis of porous TiO2 nanowires and their photocatalytic properties[J]. Front. Optoelectron., 2017, 10(4): 395-401.
[12] Tatiana I. KALGANOVA, Anna G. ORLOVA, German Yu. GOLUBYATNIKOV, Anna V. MASLENNIKOVA, Ilya V. TURCHIN. Dynamic influence of pentoxifylline on the oxygen status of Pliss’s lymph sarcoma in rat[J]. Front. Optoelectron., 2017, 10(3): 317-322.
[13] Feng GAO, Yuchun LIU, Yan XIONG, Ping WU, Bin HU, Ling XU. Fabricate organic thermoelectric modules use modified PCBM and PEDOT:PSS materials[J]. Front. Optoelectron., 2017, 10(2): 117-123.
[14] Qingshan JIANG, Ciling ZENG, Fengqiang GU, Ming ZHAO. Dynamic spot tracking system based on 2D galvanometer in free space optical communication for short distance[J]. Front. Optoelectron., 2017, 10(2): 174-179.
[15] Chao CHEN,David C. BOBELA,Ye YANG,Shuaicheng LU,Kai ZENG,Cong GE,Bo YANG,Liang GAO,Yang ZHAO,Matthew C. BEARD,Jiang TANG. Characterization of basic physical properties of Sb2Se3 and its relevance for photovoltaics[J]. Front. Optoelectron., 2017, 10(1): 18-30.
Viewed
Full text


Abstract

Cited
  Shared   
  Discussed